<p>A class of physically based models suitable for animating flexible objects in simulated physical environments was proposed earlier by the authors (1987). The original formulation works as well in practice for models whose shapes are moderately to highly deformable, but it tends to become numerically ill conditioned as the rigidity of the models is increased. An alternative formulation of deformable models is presented in which deformations are decomposed into a reference component, which may represent an arbitrary shape, and a displacement component, allowing deformation away from this reference shape. The application of the deformable models to a physically based computer animation project is illustrated.</p>